A diver needs to know the pressures of the gas tanks used during diving. A convenient way of showing the pressures to the diver is to communicate the pressures from each tank to a wearable diving computer attached to the divers' wrist, for example. In modern systems, pressure information is sent wirelessly from a tank sensor unit to the diving computer.
U.S. Pat. No. 8,275,311 disclose a method of the abovementioned kind. In the method, the pressure of a gas tank is measured and the pressure data is transmitted under water using a low first frequency to a diving computer. On the surface of the water, a second frequency higher than the first frequency is used for two-way telecommunication between the gas tank and the diving computer.
In addition, although low frequency, such as below 1 MHz, e.g. 5 kHz disclosed in U.S. Pat. No. 8,275,311 is well suitable for underwater communications but suffers from the drawback that the maximum usable bit rate and the maximum communication distance are inherently low.
U.S. RE42218 disclose an underwater alert system having a transmitter carried by a first diver and a receiver carried by a second diver. The transmitter generates a wireless alert signal to get the attention of the second diver. There are also other underwater diver-to-diver alert systems and voice and message communicating systems available. In the disclosed alert system, the receiver may identify the transmitter by an electronic identity scheme which may utilize different frequency channels or electronic identity codes. The wireless alert signal may be transmitted as ultrasonic signal, for example. However, ultrasound transmitters are large and ultrasound signals require a lot of power to be generated. These factors make the disclosed solution impractical and undesirable to be integrated to wrist-worn diving computers, for example, whose power capacity is limited and size needs to be relatively small.
Thus, there is a need for improved underwater communication systems and devices in particular for long-distance communication of signals, such as alert signals.